Hi,
Since you are using vpn, you will not need any natting. Usually the Avaya phones use the H323 ALG on the FW. Typically, people sometimes run into issues with the ALG, but if you do not require natting, you will have the option to disable the ALG altogether.
EG:
(1) How to check if ALG is running:
get alg (if you are on 5.4 and above)
ssg5-isdn-wlan-> get alg | i h323
H323 ALG : enabled
ssg5-isdn-wlan->
(2) How to disable ALG if you want to:
unset alg h323 en
There may be other hiccups depending on how the VPN is setup etc etc and it may be easier just to give it a trial run and work through the problems.